74.4电势的计算 1.点电荷的电势 E 令V=0 4户 dr E 4兀E0 q q 4兀 V=-9 q >0.V>0 兀CaF q<0,V<0q r  l  d  E  1.点电荷的电势 r r q E   3 4π 0  = 令 V = 0   =  r r l r q V   d 4π 3 0  r q V 4π 0  = dr 0, 0 0, 0     q V q V   = r r qr r 3 4π 0 d  7.4.4电势的计算